The present invention relates generally to the field of nonmetallic electrodes, and more particularly to such electrodes which are adaptable for use in electromagnetic flowmeters.
Electromagnetic flowmeters have been used extensively in the process control industry for many years. These flowmeters operate on the general principle that an electrically conductive fluid passing through a magnetic field generates an electrical signal which can be sensed by electrodes in contact with the fluid, the magnitude of the signals being related to the volumetric flow rate. However, the direct contact of the electrodes with the process fluid creates several problems.
First, the section of conduit within the flowmeter through which the fluid passes must be non-conducting. Often this is accomplished by applying to the interior of a metal conduit a liner of electrically insulating material, usually polytetrafluoroethylene (ptfe). Because the process fluids are often corrosive or are at extremely elevated temperatures, the insulating layers are carefully selected to withstand these adverse environments. However the integrity of the liner is compromised when a conventional electrode is inserted through the liner. Therefore, it is common for there to be an inadequate seal between the electrode and the insulating liner. Cracks and crevices at this interface permit the process fluid to leak past, eventually resulting in shorting of the electrode to the surrounding metal conduit. In the case of sanitary flowmeter applications, such as for dairy products, these crevices also serve to collect the fluid and promote generation of harmful bacteria.
Second, conventional electrodes are typically fabricated of corrosion-resistant metals, often a stainless steel or a noble metal, such as platinum, when stainless steel has inadequate corrosion resistance to the process environment. Despite the added expense of such metals, the corrosion resistance of the metallic electrode still may not be as effective as that of the surrounding insulating liner. Pitting and fouling are examples of problems which occur on the fluid-contacting surfaces of present metallic electrodes. The electrode and liner also have markedly different temperature coefficients of expansion. Therefore, prolonged exposure to corrosive process fluids, temperature cycling, and high temperatures tend to promote the development of crevices in the electrode-to-liner interface, aggravating the aforementioned leakage problem.
Attempts have been made to improve the integrity of the interface between an electromagnetic flowmeter electrode and the surface in which it is mounted. For example, U.S. Pat. No. 3,746,896 shows the use of an intermediate glass sealant between the electrode and the surrounding section of conduit. However, such approaches do not avoid the basic problem, namely that two dissimilar materials, such as a metal electrode and an electrical insulator, having drastically different physical characteristics, must be in intimate contact and yet maintain a fluid-tight seal while subjected to a hostile process environment. Several types of composite electrode structures, such as the carbon-containing versions disclosed in U.S. Pat. Nos. 4,337,139, 4,337,140 and 4,339,322 have found uses in other applications. However, none has been adapted to the demanding requirements of electromagnetic flowmeters.
Therefore, in view of the above, it is an object of the present invention to provide a nonporous electrode assembly having thermal and corrosion-resistance characteristics which are closely matched to those of an electrically insulating material in which it is to be mounted.
It is another object of the present invention to permit integration of such an electrode assembly into the insulating material to produce an essentially seamless interface therebetween, thereby preventing fluid leakage past the electrode.
It is yet a further object of the present invention that the electrode assembly be easily and economically adaptable to existing electromagnetic flowmeter applications.